Add TypeRegistrationDeserializer and remove BorrowedStr (#7094)

# Objective

This a follow-up to #6894, see https://github.com/bevyengine/bevy/pull/6894#discussion_r1045203113

The goal is to avoid cloning any string when getting a `&TypeRegistration` corresponding to a string which is being deserialized. As a bonus code duplication is also reduced.

## Solution

The manual deserialization of a string and lookup into the type registry has been moved into a separate `TypeRegistrationDeserializer` type, which implements `DeserializeSeed` with a `Visitor` that accepts any string with `visit_str`, even ones that may not live longer than that function call.
`BorrowedStr` has been removed since it's no longer used.

---

## Changelog

- The type `TypeRegistrationDeserializer` has been added, which simplifies getting a `&TypeRegistration` while deserializing a string.

crates/bevy_reflect/Cargo.toml

@@ -29,7 +29,7 @@ downcast-rs = "1.2"
 parking_lot = "0.12.1"
 thiserror = "1.0"
 once_cell = "1.11"
-serde = { version = "1", features = ["derive"] }
+serde = "1"
 smallvec = { version = "1.6", features = ["serde", "union", "const_generics"], optional = true }
 glam = { version = "0.22", features = ["serde"], optional = true }
 

crates/bevy_reflect/src/serde/de.rs

@@ -12,7 +12,6 @@ use serde::de::{
 };
 use serde::Deserialize;
 use std::any::TypeId;
-use std::borrow::Cow;
 use std::fmt;
 use std::fmt::{Debug, Display, Formatter};
 use std::slice::Iter;
@@ -211,13 +210,6 @@ impl<'de> Visitor<'de> for U32Visitor {
     }
 }
 
-/// Helper struct for deserializing strings without allocating (when possible).
-///
-/// Based on [this comment](https://github.com/bevyengine/bevy/pull/6894#discussion_r1045069010).
-#[derive(Deserialize)]
-#[serde(transparent)]
-struct BorrowableCowStr<'a>(#[serde(borrow)] Cow<'a, str>);
-
 /// A general purpose deserializer for reflected types.
 ///
 /// This will return a [`Box<dyn Reflect>`] containing the deserialized data.
@@ -265,6 +257,54 @@ impl<'a, 'de> DeserializeSeed<'de> for UntypedReflectDeserializer<'a> {
     }
 }
 
+/// A deserializer for type registrations.
+///
+/// This will return a [`&TypeRegistration`] corresponding to the given type.
+/// This deserializer expects a string containing the _full_ [type name] of the
+/// type to find the `TypeRegistration` of.
+///
+/// [`&TypeRegistration`]: crate::TypeRegistration
+/// [type name]: std::any::type_name
+pub struct TypeRegistrationDeserializer<'a> {
+    registry: &'a TypeRegistry,
+}
+
+impl<'a> TypeRegistrationDeserializer<'a> {
+    pub fn new(registry: &'a TypeRegistry) -> Self {
+        Self { registry }
+    }
+}
+
+impl<'a, 'de> DeserializeSeed<'de> for TypeRegistrationDeserializer<'a> {
+    type Value = &'a TypeRegistration;
+
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        struct TypeRegistrationVisitor<'a>(&'a TypeRegistry);
+
+        impl<'de, 'a> Visitor<'de> for TypeRegistrationVisitor<'a> {
+            type Value = &'a TypeRegistration;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("string containing `type` entry for the reflected value")
+            }
+
+            fn visit_str<E>(self, type_name: &str) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                self.0.get_with_name(type_name).ok_or_else(|| {
+                    Error::custom(format_args!("No registration found for `{type_name}`"))
+                })
+            }
+        }
+
+        deserializer.deserialize_str(TypeRegistrationVisitor(self.registry))
+    }
+}
+
 struct UntypedReflectDeserializerVisitor<'a> {
     registry: &'a TypeRegistry,
 }
@@ -280,14 +320,9 @@ impl<'a, 'de> Visitor<'de> for UntypedReflectDeserializerVisitor<'a> {
     where
         A: MapAccess<'de>,
     {
-        let type_name = map
+        let registration = map
-            .next_key::<BorrowableCowStr>()?
+            .next_key_seed(TypeRegistrationDeserializer::new(self.registry))?
-            .ok_or_else(|| Error::invalid_length(0, &"at least one entry"))?
+            .ok_or_else(|| Error::invalid_length(0, &"at least one entry"))?;
-            .0;
-
-        let registration = self.registry.get_with_name(&type_name).ok_or_else(|| {
-            Error::custom(format_args!("No registration found for `{type_name}`"))
-        })?;
         let value = map.next_value_seed(TypedReflectDeserializer {
             registration,
             registry: self.registry,

crates/bevy_scene/src/serde.rs

@@ -1,7 +1,10 @@
 use crate::{DynamicEntity, DynamicScene};
 use anyhow::Result;
 use bevy_reflect::serde::{TypedReflectDeserializer, TypedReflectSerializer};
-use bevy_reflect::{serde::UntypedReflectDeserializer, Reflect, TypeRegistry, TypeRegistryArc};
+use bevy_reflect::{
+    serde::{TypeRegistrationDeserializer, UntypedReflectDeserializer},
+    Reflect, TypeRegistry, TypeRegistryArc,
+};
 use bevy_utils::HashSet;
 use serde::ser::SerializeMap;
 use serde::{
@@ -9,7 +12,6 @@ use serde::{
     ser::SerializeStruct,
     Deserialize, Deserializer, Serialize, Serializer,
 };
-use std::borrow::Cow;
 use std::fmt::Formatter;
 
 pub const SCENE_STRUCT: &str = "Scene";
@@ -353,15 +355,16 @@ impl<'a, 'de> Visitor<'de> for ComponentVisitor<'a> {
     {
         let mut added = HashSet::new();
         let mut components = Vec::new();
-        while let Some(BorrowableCowStr(key)) = map.next_key()? {
+        while let Some(registration) =
-            if !added.insert(key.clone()) {
+            map.next_key_seed(TypeRegistrationDeserializer::new(self.registry))?
-                return Err(Error::custom(format!("duplicate component: `{key}`")));
+        {
+            if !added.insert(registration.type_id()) {
+                return Err(Error::custom(format_args!(
+                    "duplicate component: `{}`",
+                    registration.type_name()
+                )));
             }
 
-            let registration = self
-                .registry
-                .get_with_name(&key)
-                .ok_or_else(|| Error::custom(format!("no registration found for `{key}`")))?;
             components.push(
                 map.next_value_seed(TypedReflectDeserializer::new(registration, self.registry))?,
             );
@@ -385,13 +388,6 @@ impl<'a, 'de> Visitor<'de> for ComponentVisitor<'a> {
     }
 }
 
-/// Helper struct for deserializing strings without allocating (when possible).
-///
-/// Based on [this comment](https://github.com/bevyengine/bevy/pull/6894#discussion_r1045069010).
-#[derive(Deserialize)]
-#[serde(transparent)]
-struct BorrowableCowStr<'a>(#[serde(borrow)] Cow<'a, str>);
-
 #[cfg(test)]
 mod tests {
     use crate::serde::{SceneDeserializer, SceneSerializer};